As loads without an earth point, there are cold-cathode tube and so on, and it is necessary to adjust a voltage applied to such a load to be constant. In consequence, in order to detect a load current which flows into a load, an insulating component represented by a current transformer or a photocoupler is required. That is, it is necessary that a piezoelectric transformer driving apparatus keeps an insulating state to a load circuit without an earth point.
FIG. 4 is an explanatory diagram explaining current detection of a balanced output using a current transformer in a conventional example (patent document 1: Japanese Patent Laid-Open No. 2001-85759). In FIG. 4, reference numeral 101 denotes a signal source, reference numeral 102 denotes a load, reference numeral 103 denotes a current transformer, and reference numeral 104 denotes a resistor. One terminal of the signal source 101 is connected to one terminal of the load 102, and the other terminal is connected to the other terminal of the load 102 through the current transformer 103. In addition, one terminal of a secondary side of the current transformer 103 is grounded.
If a turn ratio of a primary side and a secondary side of the current transformer 103 is 1:n, if a voltage of the resistor 104 connected to the secondary side is Vd, and if a load current which flows into the load 102 is Io, then the relation of the load current Io is expressed by the following formula 1.Vd=R·Io/n  formula 1
Hence, the load current Io is expressed by the following formula 2.Io=n·Vd/R  formula 2In addition, in formulas 1 and 2, R is a value of the resistor 104. The load current Io is detected from formula 2. On the basis of the detected load current Io, a frequency and a voltage of an output of the signal source 101 are controlled, and a voltage applied to the load 102 is adjusted automatically.
However, in the conventional example mentioned above, in order to keep an insulating state to the load 102, the current transformer 103 which is an insulating component is used. In addition, there are some which use a photocoupler instead of the current transformer 103. There is a problem that use of these insulating components results in an increase of component costs and manufacturing steps, and leads to a cost increase of a product.
The present invention aims at providing a piezoelectric transformer driving apparatus which solves the above-mentioned problem, and not only drives a load balanced to the ground, but also makes insulating components, such as a current transformer and a photocoupler, unnecessary, and can detect a load current.